Journal of Photochemistry and Photobiology A: Chemistry
Fundamental insights into the oxidation of lignocellulosics obtained from singlet oxygen photochemistry
Introduction
The aspect of harnessing photochemical oxidation reactions as opposed to traditional ground state or thermal chemical reactions for targeting lignin depolymerization reactions has received a considerable amount of attention within the last 10 years [1], [2], [3], [4], [5]. This attention derives in part from environmental regulations that seek to minimize the application of chlorine and chlorine-containing bleaching chemicals for the bleaching of kraft pulp. Additionally, photodelignification is a very novel and powerful approach to target lignin residues since the photochemical interactions leading to delignification can be selectively tuned by the appropriate choice of light energy. Photodelignification strategies rely principally on generating reactive species, such as hydroxyl radicals that can hydrolyze the generally unreactive ether bonds found between monomeric phenyl propanoid units in the three-dimensional structure of lignin. A fairly robust oxidant that has recently seen success for chemically degrading the lignin superstructure is singlet oxygen [6].
Marcoccia and his coworkers reported that irradiation of a slurry of kraft pulp with UV light in the presence of oxygen under very mild conditions reduced the lignin content of the pulp. It was found that this delignification was strongly dependent on the process conditions, such as the concentration of oxidant and substrate and the intensity and wavelength of the irradiating light [7]. It was known that the UV irradiation of ground state oxygen generated various reactive species. Singlet oxygen is among these species that can be generated via a stepwise photochemical process. In general, photochemical conversion processes that employ a photosensitizer for the transfer of excitation energy to the triplet ground state of oxygen are well established [8], [9].
Singlet oxygen is an excited state of molecular oxygen with pronounced electrophilic character, hence reacting well with electron-rich groups such as olefinic or aromatic derivatives [6], [10]. These electron rich groups tend to form an intermediate exciplex as a result of charge transfer reactions between the electron-rich substrate and the singlet oxygen. This exciplex is able to later form dioxetanes, hydroperoxides, or endoperoxides [11]. Shown in Fig. 1 is an example of the reactivity of singlet oxygen with a lignin-like monomeric unit that typically possesses aromatic character.
In our studies, we examined the influence of singlet oxygen on the delignification and carbohydrate chemistry of two types of kraft softwood pulp, one having an increased level of condensed phenolic structures versus a “control.” We were particularly interested in determining if there was an enhancement in the efficacy of delignification versus carbohydrate damage due to the highly electrophilic nature of singlet oxygen.
Section snippets
Substrates and preparation for photolysis
All lignocellulosic substrates were commercially obtained from a sponsor of the Institute of Paper Science and Technology at the Georgia Institute of Technology. The substrates were derived from kraft pulping of loblolly pine (P. taeda) whose final product contained 4% by mass of residual lignin with the remainder being carbohydrates, principally cellulose. The approximate degree of the polymerization of the carbohydrate component was determined using the accepted cupriethylene diamine (CED)
Control experiments and depolymerization of lignin and carbohydrates
Control experiments were done to determine the contribution of individual variables to the photodelignification response of the substrate under study. Shown in Fig. 2 is the delignification response of the substrate as a function of the variables. It is not surprising that heat removes approximately 20% of the lignin available since leaching studies have shown this to be the case [14]. The spectral output of the lamp only causes an additional 14% delignification to occur. Again, this result is
Conclusion
This study has investigated the reactivity of singlet oxygen with kraft softwood substrates with respect to the chemistry of the lignin and the cellulose. It was determined that despite the relatively high selectivity of singlet oxygen for lignin aromatic units, the degradation of the cellulose nevertheless occurs after approximately 50% removal of the lignin. The most salient difference between this system and a typical ground state oxygen delignification system is the absence of condensed
Acknowledgements
We gratefully acknowledge the sponsors of the Institute of Paper Science and Technology Center at the Georgia Institute of Technology who have helped to make this work possible. We are also indebted to Professor C. Foote of UCLA whose insightful advice propelled this work.
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